1. Field of the Invention
The present invention relates to a reticle manufacturing method.
2. Description of the Related Art
With the progress of fine patterns on account of the higher integration of the semiconductor device such as LSI, the super-resolution exposure technology such as the phase shift method is demanded. There are various phase shift methods. Out of them, the exposure method using a chromeless reticle can form the finest pattern. For this reason, such exposure method is expected to contribute largely to the miniaturization of the semiconductor devices. However, since the method of manufacturing the chromeless reticle becomes recently extremely complicated, the above exposure method using the chromeless reticle makes worse the productivity of the semiconductor device and cannot meet the demand for the mass production of the semiconductor device.
Subsequently, a chromeless reticle manufacturing method in the prior art will be explained with reference to FIGS. 1A to 1E hereunder.
FIGS. 1A to 1E are sectional views showing a chromeless reticle manufacturing method in the prior art in order of steps.
At first, as shown in FIG. 1A, a light shielding layer 2 made of chromium and a resist 3 are formed sequentially on a quartz substrate 1.
Then, steps required for obtaining the sectional structure showing in FIG. 1B will be explained hereunder.
First, a first resist pattern 3c having a wide resist portion 3a and a narrow resist portion 3b is formed by exposing the resist 3 by means of the electron beam (EB) exposure machine and then developing the resist 3. The Cr (chromium) light shielding layer 2 and the glass substrate 1 are then etched by using the first resist pattern 3c as a mask. Thus, the light shielding layer 2 is patterned to form a wide light shielding portion 2a and a narrow light shielding portion 2b. Moreover, the portion of the quartz substrate 1, where the portions 2a and 2b does not cover, is thinned by the etching and thus a thin thickness portion 1c is formed. A wide convex pattern 1a and a narrow convex pattern 1b, each having original thickness of the substrate 1 without being subjected to the etching, are formed under the portions 2a and 2b, An etching depth of the thin thickness portion 1c is determined according to a wavelength of an exposure light that is applied to the completed chromeless reticle. The etching depth is set to such a thickness that a phase of the light passed through the thin thickness portion 1c is sifted by just π from the phase of the light passed through the convex patterns 1a, 1b. 
After the etching, the first resist pattern 3c is removed.
Then, as shown in FIG. 1C, the resist is coated on the overall surface and then exposed/developed by the electron beam exposure machine. Thus, a second resist pattern 4 having a planar shape that is smaller than the wide light shielding portion 2a is selectively formed only on the wide light shielding portion 2a. 
Then, as shown in FIG. 1D, the light shielding portions 2a, 2b are etched by using the second resist pattern 4 as a mask. Thus, side surfaces of the wide light shielding portion 2a retreats, and thus an upper surface of the wide convex pattern 1a is exposed by an amount of such retreat. Also, the narrow light shielding portion 2b on the narrow convex pattern 1b is removed.
After that, as shown in FIG. 1E, a basic structure of the chromeless reticle in the prior art is completed by removing the second resist pattern 4.
FIG. 2 is a view showing a sectional shape of the chromeless reticle in the prior art together with a graph showing an intensity of the exposure light on the wafer.
As shown in FIG. 2, the chromeless reticle has a large width area A in which the wide light shielding portion 2a remains for exposing a wide pattern onto the wafer, and a small width area B in which the chromium pattern is removed for exposing a finer pattern than the large width area A.
In the small width area B, phases of the lights, each passed through the narrow convex pattern 1b and the thin thickness portion 1c, are shifted by π. Therefore, both lights interfere and cancel each other at the bottom of the side surface (edge) of the narrow convex pattern 1b. As a result, an intensity of the exposure light is sharply changed at the bottom of the edge and thus a fine and sharp pattern can be obtained on the wafer. The phenomenon that the intensity of the light is emphasized at the edge portion in this manner is also called the edge contrast effect.
According to this edge contrast effect, as disclosed in FIGS. 2a) to b) in Non-Patent Literature 1, the exposure light is not canceled around the center of the pattern if the width of the convex pattern is excessively wide, and therefore a desired dark pattern cannot be formed. As such, the width of the narrow convex pattern 1b must be reduced narrower than a certain upper limit value to achieve the edge contrast effect. This upper limit value is referred to as Cmax in the following. In the example shown in FIG. 2, the width of the narrow convex pattern 1b is set just to this Cmax. According to this, the edge contrast can be obtained only for the region that goes inward by Cmax/2 from the edge of the pattern.
In contrast, in the large width area A in FIG. 2, the upper surface of the wide convex pattern 1a is exposed by retreating the side surfaces of the wide light shielding portion 2a from the side surfaces of the wide convex pattern 1a, while leaving partially the wide light shielding portion 2a. As a result, most of the exposure light is shielded by the wide light shielding portion 2a and also the contrast of the pattern on the wafer is improved by the edge contrast effect.
The technology relevant to the present invention is disclosed on Patent Literatures 1, 2.                [Patent Literature 1] Patent Application Publication (KOKAI) Sho 64-21450        [Patent Literature 2] Patent Application Publication (KOKAI) Hei 4-85814        [Non-Patent Literature 1] H. Iwasaki, et al., “Fabrication of the 70-nm line patterns with ArF chromeless phase-shift masks”, SPIE, vol. 4754        
Meanwhile, an amount of retreat of the wide light shielding portion 2a in the large width area A must be set smaller than Cmax/2 to achieve the edge contrast effect. However, if the amount of such retreat is set excessively small, intensity of exposure light passing through the wide convex pattern 1a is reduced and thus the exposure light passed through the wide convex pattern 1a cannot uniformly interfere with the exposure light passed through the thin thickness portion 1c, which in turn makes the edge of the pattern dim and lowers the pattern contrast.
For this reason, in the chromeless reticle of this type, the brightness contrast of the exposure light passed through the reticle must be set equal under the both edges by retreating the both edges of the wide light shielding portion 2a from the side surfaces of the wide convex pattern 1a by the same distance.
However, in the above chromeless reticle manufacturing method in the prior art, the second resist pattern 4 for forming the wide light shielding portion 2a must be formed apart from the first resist pattern 3c for forming the wide convex pattern 1a. Therefore, an alignment between the wide convex pattern 1a and the second resist pattern 4 must be performed precisely.
If this alignment is imprecise, the center of the wide light shielding portion 2a is displaced from the center of the wide convex pattern 1a, as shown in FIG. 3, and a intensity of exposure light becomes different at both edges of the wide light shielding portion 2a and thus the brightness contrast becomes different at the bottoms of both edges.
However, according to the patterning performed by the electron beam exposure machine, an alignment precision between the wide convex pattern 1a and the second resist pattern 4 (a minimum value of the displacement between both centers of the wide convex pattern 1a and the second resist pattern 4) is 20 to 30 nm at best. Therefore, it is difficult to align the wide convex pattern 1a and the second resist pattern 4 with good precision and thus it is difficult to maintain the balance of the contrast between the exposure lights as described above. This problem appears not only in the electron beam exposure machine but also in the exposure machine such as the stepper, or the like.
In addition, if the resist patterns are formed twice in this manner, the exposure date must be prepared for each of the patterns, which in turn prolongs the manufacturing steps and raises cost, so the above method is unsuitable for the mass production.